The invention relates to a light-sensitive semiconductor component that includes a channel region with a first type of doping and zones with an opposed type of doping that are in contact therewith, a pn junction being formed at the area of contact between the channel region and the zones.
A semiconductor component of the kind set forth is known from EP 0 883 187 A1. The component is constructed in principle as a semiconductor diode in CMOS technology. On a p doped substrate therein there is provided a p- (that is, weaker) doped channel region in which a xe2x80x9cdot-shapedxe2x80x9d, or slightly laterally expanded, n doped zone is inserted by diffusion or preferably by implantation. This n doped zone will be referred to hereinafter as a xe2x80x9cdot zonexe2x80x9d because of its shape relative to the channel region. A pn junction is formed in known manner between the n doped dot zone and the p doped channel region.
The known semiconductor diode is connected so as to detect light quanta. A light quantum that is incident in the laterally extending channel region generates a pair of charge carriers therein, that is, a hole and a free electron. The charge carriers exist until they are xe2x80x9cdestroyedxe2x80x9d again by recombination. When the doping of the channel region is weak enough, however, they will have a long life during which notably the minority charge carriers (for example, an electron in the p region) can travel a comparatively large distance under the influence of diffusion processes. The minority charge carrier can then notably reach the barrier layer between the p doped channel region and the dot zone, said minority carrier then being pulled into the dot zone because of the electrical field prevailing at that area. A substantial part of the minority charge carriers of the channel region that are produced by light quanta thus accumulates in the dot zone in which their presence can be detected by means of appropriate electronic evaluation circuitry.
As opposed to conventional light-sensitive semiconductor diodes with a planar pn junction, the dot zone in the diode disclosed in EP 0 883 187 A1 does not extend over a large area in the channel region, but is rather limited to a minimum part of the surface area of the channel region. This is due to the fact that the remaining surface area over the channel region should be reserved so as to accommodate integrated electronic evaluation circuitry for reading out the charge of the semiconductor diode. Therefore, this surface area over the p doped channel region is provided with n++ doping so as to create a basis for said electronic circuitry that is electrically insulated from the channel region. The size of a pixel, that is, the surface area of a light-sensitive semiconductor component that can be selectively read out by the electronic circuitry, is limited by the diffusion length of the charge carriers in the p doped channel region in the diode disclosed in EP 0 883 187 A1. This is because only minority charge carriers that can reach a dot zone via diffusion can be detected in this dot zone. Therefore, only comparatively small pixels can be realized. Furthermore, because of the undirected nature of the diffusion, crosstalk of the signals occurs between neighboring pixels in a component as disclosed in EP 0 883 187 A1.
For many applications it is desirable to have available light-sensitive semiconductor components with pixels of comparatively large area. An example in this respect is an optical detector for reading out X-ray scintillators. The detection of X-rays usually takes place in two stages where an X-ray quantum first causes the emission of visible light in a scintillator crystal and this visible light is subsequently detected by a light-sensitive semiconductor diode in a second step. It is a typical aspect of this process that it is necessary to detect very small quantities of light that, moreover, are distributed across a large area in space because of the undirected emission. For example, the pixel of a semiconductor detector for an X-ray scintillator typically has a surface area of 2 mm2 and the entire detector is then usually composed of a number of such pixels and has a format of, for example 4xc3x97100 cm2 for a typical application.
It is known that light-sensitive semiconductor components with integrated amplifier circuits exhibit noise which increases as the capacitance of the semiconductor diode increases. Therefore, the reduction of the capacitance of detector elements is a crucial condition in realizing low-noise optical front ends that include a light-sensitive element and an amplifier. In this respect it is to be noted that the component is advantageously manufactured by means of customary integrating semiconductor techniques, for example, CMOS techniques, that define numerous parameters of the component for technical reasons. This holds notably for the doping strength of the channel region so that, generally speaking, it is no longer available for adaptation of the capacitance of the semiconductor diode.
Considering the foregoing it was an object of the present invention to provide a light-sensitive semiconductor element for an X-ray detector in an X-ray examination apparatus that has a comparatively large pixel surface area in conjunction with a low capacitance and hence is suitable in particular for use in X-ray scintillators. It should preferably be possible to manufacture the semiconductor component in standard (CMOS) production steps and the component should also have a high radiation hardness in respect of ionized X-rays, a suitable channel separation and an exactly defined geometry.
This object is achieved by means of a semiconductor component as disclosed in the characterizing part of claim 1. Advantageous further embodiments are disclosed in the dependent claims.
The semiconductor component thus includes a channel region with a first type of doping as well as dot zones that are in contact with the channel region and have a type of doping that opposes the doping of the channel region. The channel region may thus be p doped and the dot zones may be n doped or vice versa. Because of such different doping, a respective pn junction arises in the area of contact between the channel region and a dot zone, that is, a semiconductor diode. Furthermore, the semiconductor component includes at least one group of several dot zones, the dot zones of said group being connected electrically in parallel with one another.
Because of the electrical connection of the dot zones of a group, the charge accumulated in the dot zones can be uniformly read out. This means that the dot zones of the group together constitute a pixel. The size of this pixel is then determined by the surface area across which the group of dot zones is distributed; it can quasi be made arbitrarily large. Preferably, many of such groups, each time constituting a respective pixel, are arranged on the semiconductor component so as to cover the surface area thereof.
Preferably, the dot zones of a group are arranged so as to be coherent. This means that each dot zone of the group has at least one neighboring dot zone that is also a member of the same group. The surface area that is covered by the dot zones is typically bounded so as to be rectangular or compact in a different way.
Furthermore, the dot zones in the channel region are preferably distributed according to a hexagonal pattern. This means that each dot zone is situated at the center of a hexagon whose corners are occupied by dot zones. The smallest geometrical cell of this arrangement is an equilateral triangle with dot zones provided at its corners.
The formation of a pixel from hexagonally arranged and electrically coupled dot zones results in a detector element that has a substantially reduced capacitance and at the same time a suitable sensitivity across the entire pixel surface that is occupied by the group. As opposed to conventional detector diodes, the barrier layer (on whose surface area the capacitance of the diode is approximately proportionally dependent) does not extend across the entire pixel surface, but only across small dot-shaped areas. Nevertheless, practically all charge carriers that are produced by the incidence of light on the pixel surface area detected, because from areas of the channel region outside the barrier layer they can diffuse into the barrier layer in which they are diverted to the dot zone. Because of the hexagonal arrangement of the dot zones on the pixel surface, it can then be ensured that optimum draining of the minority charge carriers produced can take place while using an as small as possible overall surface area or number of dot zones. This means that the capacitance of the pixel is low while at the same time practically all light that is incident on the pixel surface is detected.
The electrical connection of the dot zones of a group is preferably realized in such a manner that each dot zone is connected to a collecting lead via at least two current paths. This can be realized notably by way of a plurality of electrical leads that extend in parallel and are provided on the semiconductor component in a customary manner, said leads being electrically connected to a collecting lead at the oppositely situated edges of the surface that is occupied by the group. The configuration of the leads is preferably chosen to be such that all dot zones can be electrically connected to the collecting lead while using an as small as possible quantity of metal, thus ensuring that the detector surface that is lost to the detection of light due to the masking by metal is as small as possible. The double or multiple connection of the relevant leads to a collecting lead that extends around the pixel surface offers the advantage of redundancy, so that in the event of a rupture in the lead in one location the charges of all dot zones that are connected to the lead can still be conducted to the collecting lead.
In a further embodiment of the invention the dot zones of a group (of a pixel) are surrounded by a circumferential guard ring which is formed as an opposed and preferably deeper doping in the channel region. The guard ring thus forms a line-shaped pn junction in the channel region which encloses the active surface of the pixel and thus defines the weakly doped channel regions of the individual pixels. This can be ensured notably by deep doping of the guard ring (preferably by diffusion) as well as by application of a counter voltage to the guard ring. The guard ring thus ensures that the various pixels are strictly separated from one another and that minority charge carriers that are generated in the surface of one pixel do not travel to the other pixel in which they are detected.
The dot zones in the vicinity of a guard ring are preferably arranged in a denser packing. In particular further dots can be added at a constant distance from the guard ring in order to achieve uniform sensitivity in the edge zone of the pixel. The guard ring and the additional dot zones additionally improve the frequency behavior of the detector, because the slow diffusion of photogenerated charge carriers from neighboring regions is prevented.
The surface of the channel region between the dot zones preferably is provided with a so-called threshold voltage implantation. Such a threshold voltage implantation is of the same type of doping (p or n) as the channel region on which it is formed and is generally provided in all CMOS semiconductor diodes in order to suppress the so-called xe2x80x9cweak inversionxe2x80x9d. In the light-sensitive semiconductor element in accordance with the invention a threshold voltage implantation has the positive effect that the doping strengths that vary in the direction perpendicular to the surface of the channel region cause electric fields which support a laterally directed diffusion of charge carriers. The life of minority charge carriers generated by light is thus prolonged, thus increasing the chances of detection in a dot zone.
The channel region is preferably formed in such a manner that minority charge carriers that are generated therein by light have an as large as possible diffusion length. This length is typically in the range of from 50 to 200 xcexcm. Large diffusion lengths increase the probability that a minority charge carrier by chance reaches the area of a barrier layer or a dot zone by diffusion so that it is detected. The diffusion length can be influenced during the formation of the channel region, for example, by way of weak doping, less implantation, fewer process steps and so on.
In order to adjust a sufficiently high yield of detected minority charge carriers from the neighboring channel region, the dot zones are preferably arranged at a distance from one another, said distance being of the order of magnitude of the diffusion length of the minority charge carriers in the channel region. This means that this distance amounts to from approximately 10 to 200% of the diffusion length.
The surface area of the preferably circular or rectangular dot zones is preferably formed with the minimum structure size that is permitted by the process used. The surface area typically amounts to from 1 to 5 xcexcm2, preferably approximately 2 xcexcm2. Such a size of the surface area on the one hand still enables adequate contacting of the dot zones by (metallic) leads while on the other hand the surface area is sufficiently small so as to ensure the desired minimized capacitance.
The dot zones of a group (pixel) are preferably arranged in such a manner that they fill an approximately rectangular region whose surface area amounts to from approximately 0.5 to 5 mm2, preferably approximately 1 mm2. Such large surface areas for optical semiconductor components are required notably for the pixels of X-ray detectors in order to enable as complete as possible detection of the light that is generated in a scintillator crystal. Using the present invention, despite the large surface area sufficiently small capacitances of the semiconductor diode can be ensured, so that a sensitive detector arrangement is realized that can be read out with a small amount of noise.
The CMOS technique that is an industry standard and is suitable for integration of the electronic circuitry is preferably used for the manufacture of the described semiconductor element. As a result of the hexagonal arrangement of electrically interconnected dot zones, a reduction of the capacitance of the diode detector thus formed is also possible in cases where the detector for the remainder is manufactured in conformity with standard CMOS parameters that define notably the doping strengths of channel regions.
The object is also achieved by means of an X-ray detector with a light-sensitive semiconductor component as claimed in claim 14.
Moreover, the object is also achieved by means of an X-ray examination apparatus that includes an X-ray detector and a light-sensitive semiconductor component as claimed in claim 15.